1. Field of the Invention
The present invention relates to an IC card and an IC card system and, more particularly, to an IC card which transmits in a non-contact manner a signal modulated with desired data and to an IC card system using the IC card.
2. Description of the Related Art
IC card systems using IC cards have been applied to ticket examination systems for use in transport facilities, room entry/exit control systems, etc. An IC card system of this kind has been proposed which is constituted of IC cards carried by users and a read/writer (i.e., IC card processor) for exchanging various sorts of data with each of the IC cards. Various sorts of data are transmitted and received between the IC card and the read/writer in a non-contact manner.
That is, in this kind of IC card system, the read/writer forms a transmission signal by modulating a carrier having a predetermined frequency with a desired data sequence, and sends out the transmission signal to the IC card via an antenna.
The IC card receives the transmission signal via an antenna and demodulates the transmission signal to reproduce the data sent from the read/writer. Further, the IC card sends out data such as personal information stored therein to the read/writer in response to the received data by modulating a predetermined carrier with the data to be transmitted.
The read/writer receives the data sent from the IC card, and opens or closes a door of a ticket examination machine or allows entry into or exit from a room.
A type of IC card for such use has been proposed which is arranged to send out desired data to a read/writer by the method of changing the power radiated from the antenna by changing the load impedance of the antenna (hereinafter referred to as "load switching system").
That is, in the antenna of the IC card, a transmission signal sent from the read/writer is induced and this transmission signal is reradiated. If at this time the load impedance of the antenna is changed, the reradiated power is correspondingly changed. Then, in the receiving antenna coupled to the antenna of the IC card, the carrier of the transmission signal is detected at the signal level corresponding to the change in the load impedance.
In such a load switching system, desired data can be sent out by being modulated in an amplitude shift keying (ASK) manner based on a simple arrangement. The overall configuration can be correspondingly simplified.
This kind of IC card is carried by a user and is brought close to the read/writer when used. Therefore, the signal level of the transmitted signal induced in the antenna varies largely depending upon use.
In the load switching system, therefore, there is a need to use a semiconductor device having a sufficiently high withstand voltage as a semiconductor device for changing the load impedance of the antenna to ensure the desired operation even when the signal level varies largely as described above. If a semiconductor device having such a sufficiently high withstand voltage is used, it is difficult to integrate the entire circuit of the IC card to the desired degree of integration.
As a solution for this problem, a method of processing the signal obtained from the antenna by clamping the signal may be used. If such a method is used, the power reradiated by following the change in the load impedance cannot be sufficiently changed. That is, the degree of amplitude shift keying modulation is reduced, resulting in a deterioration in data quality.